Last year for Spring Break, my family and my brother’s family went to Bentonville. It was a great location enough non-cycling to do and great riding for us and my 16 yo son. Previously I rode in Phoenix when there for a soccer tournament for my daughter. It would meet the criteria of decent riding and other stuff to do. Where else should we consider? Phoenix is a direct flight from Philly as was Bentonville.
edit: Spring Break is first week of April, so needs to be somewhere warmish.
Places I would consider that I haven’t ridden yet:
Park City, UT - direct flight to SLC then an easy drive, and plenty of non-mtb stuff to do.
North Carolina - Brevard/Asheville/Pisgah
Places I have been that I would recommend:
Vermont - could be a little tricky in May depending on how the winter goes, but you could fly into Burlington and stay somewhere like Stowe. Cool town with plenty to do and easy access to a ton of good riding (Cady Hill in Stowe, Perry Hill and Mad River to the south, Kingdom Trails 90 minutes away).
Vancouver Island BC - tougher travel logistics, especially if you’re bringing your own bikes, but it’s worth it once you get out there. Specifically Cumberland and Campbell River were my favorite spots, but there are lots.
St. George UT - I haven’t mountain biked there but it’s a cool town and the riding is supposed to be great. Plenty to do and an easy drive from Vegas.
Vermont and the Kingdom Trails are on the list for next Summer. But spring break is early this year. First week of April. Sedona was suggested by my wife.
Moab is a great choice, perfect weather that time of year and plenty of biking options, as well and nationals parks and a nice town.
May is almost certainly too early for Vermont; we don’t dry out from the winter quickly enough up here in northern New England for the most part, so you wouldn’t want to plan a trip around the variability of conditions and possibility of trail closures.